The North Atlantic Subtropical Gyre (NASTG) has experienced the fastest expansion of oligotrophic waters worldwide in response to ocean warming. We study the trophic regime changes in the NASTG by using 21 years (1998–2018) of satellite chlorophyll‐a (CHL) data, complemented with other variables such as Sea Surface Temperature, optical backscatter coefficients (b_bp), Secchi disk (z_SD), and Mixed Layer Depth. To this aim, we describe the spatial/temporal variability of waters with the lowest CHL concentrations (CHL ≤ 0.04 mg m⁻³) with an inter‐annual variability analysis of key environmental variables. Main results demonstrate that these ultra‐oligotrophic waters are spatially expanding and increasing in frequency, shifting the NASTG to a dominant quasi‐permanent ultra‐oligotrophic condition, thus confirming the ongoing ocean desertification.

In the last few decades, phytoplankton biomass has been commonly studied from space. However, satellite analysis of non‐algal particles (NAPs), including heterotrophic bacteria and viruses, is relatively recent. In this work, we estimate the backscattering coefficient associated with the NAP fraction that does not covary with chlorophyll based on satellite particulate backscattering coefficient and chlorophyll (b_bpNAP). b_bpNAP is computed at 100‐km resolution using 19 years of monthly satellite data. We find clear differences in b_bpNAP between northern and southern oceans. High b_bpNAP values are found in the Arctic and Southern Oceans, the North Atlantic area influenced by the Gulf Stream current, as well as shelf regions (i.e., Patagonian shelf) affected by upwelling regimes. Low correlation between chlorophyll and backscattering prevents precise b_bpNAP estimations in oligotrophic areas (e.g., subtropical gyres). These b_bpNAP estimations lead to a reduction to half in satellite‐based phytoplankton biomass estimates respect to previously published results.

Marine heatwaves (MHWs) represent anomalously warm temperature conditions of seawater that may affect marine life and ocean biogeochemistry. Under such conditions, phytoplankton communities may modify their structure and functions, and their resilience is not assured. This study characterizes the impact of MHWs on the phytoplankton spring bloom in the North‐Western Mediterranean Sea. Here, we synergistically combine autonomous observations from BioGeoChemical‐Argo floats, satellite‐based and marine ecosystem model data, and show that MHW events occurring during winter drastically inhibit phytoplankton carbon biomass in spring by up to 70%. Such reduction is related to the enhanced stratification of the water column under MHWs which hinders the renewal of nutrients from deep‐ocean reservoirs, thus preventing surface phytoplankton from blooming. This process negatively impacts particulate organic carbon stocks within the mixed layer, while severe events cause an earlier shift of phytoplankton phenology that provokes changes in zooplankton biomass distribution.

The ways in which marine biological activity affects climate, by modifying aerosol properties, are not completely understood, causing high uncertainties in climate predictions. In this work, in situ measurements of aerosol chemical composition, particle number size distribution, cloud condensation nuclei (CCN), and ice‐nucleating particle (INP) number concentrations are combined with high‐resolution sea surface chlorophyll‐a concentration (CHL) and back‐trajectory data to elucidate the relationship between oceanic biological activity and marine aerosol. The measurements were performed during an intensive field campaign conducted in late summer (August–September) 2015 at the Mace Head Research Station (MHD). At the short time scale (1–2 months) of the experiment, we observed a clear dependency of the main aerosol physicochemical and cloud‐relevant properties on the patterns of biological activity, in specific oceanic regions with a delayed response of about 1–3 weeks. The oceanic region comprised between 47°–57°N and 14°–30°W was identified as the main source of biogenic aerosols during the campaign, with hints of some minor influence of waters up to the Greenland coast. These spatial and temporal relationships demonstrate that the marine biota influences aerosol properties under a variety of features up to the most cloud‐relevant properties. Such dependency of aerosol properties with oceanic biological activity was previously reported over the North Atlantic Ocean only for multiyear data sets, where the correlation may be enhanced by coincident seasonalities. A better knowledge of these short time scale interactions may lead to a significant improvement in understanding the ocean‐atmosphere‐cloud system, with important impacts on climate science.

Particulate optical backscattering (b_bp) is a crucial parameter for the study of ocean biology and oceanic carbon estimations. In this work, b_bp retrieval, by the quasi-analytical algorithm (QAA), is assessed using a large in situ database of matched b_bp and remote-sensing reflectance (R_rs). The QAA is also applied to satellite R_rs (ESA OC-CCI project) as well, after their validation against in situ R_rs. Additionally, the effect of Raman Scattering on QAA retrievals is studied. Results show negligible biases above random noise when QAA-derived b_bp is compared to in situ b_bp. In addition, R_rs from the CCI archive shows good agreement with in situ data. The QAA’s functional form of spectral backscattering slope, as derived from in situ radiometry, is validated. Finally, we show the importance of correcting for Raman Scattering over clear waters prior to semi-analytical retrieval. Overall, this work demonstrates the high efficiency of QAA in the b_bp detection in case of both in situ and ocean color data, but it also highlights the necessity to increase the number of observations that are severely under-sampled in respect to others environmental parameters.